Process of producing alpha phosphate coating on metal



.l siteniieid EQBQ entree stares earner camera MATTHEW GREEN AND VAN M. DAE$EY,

PARKER RUST-PROOF COMPANY, OE DETRQIL'T, MICHIGAN, A COBPORATIQN 03E MICHIGAN OF DETROIT, MICHIGAN, A SSIGNORS TO 'BROCESS OF PBQDUGING: A PHOSPHATE COATING 0N METAL No Drawing.

This invention relates to producing a phosphate coating on metal and more particularly to a method by which the desired coating can be produced quickly and economically.

The coating of iron and steel by subjection to a dilute solution of acid phosphates and phosphoric acid has been in commercial use for many years. It is customary to prepare a phosphate bath and when properly balanced, to immerse the articles to be coated in the heated bath until complete or sufficient reaction has taken place. The action" consists essentially in the combination of some of themetal on the surface of the article with phosphoric acid or acid phosphates and the forming in situ on the surface of the metal of normal or substantiall insoluble phosphates. This action is normal y accompanied by the release of hydrogen, and the escape of the hydrogen bubbles is some indication of the activity of the reaction. For a rust-proof coating ascomplete as the bath will form, the articles are left in the bath until reaction, a indicated by the escape of hydrogen bubbles, has substantially ceased.

The phosphates in the bath are depleted to some extent in producing the phosphate coating upon the articles, but a large proportion of the phosphates remain in the bath after the first batch has been coated to completion, and generally a number of loads of work may be cbated in succession in the same bath without replenishment. Even when the bath is so depleted that further use without replenishment is inadvisable, there still remains in the bath a large quantity of chemicals which it is inadvisable to waste. For this reason it is customary to add fresh chemicals to the bath from to time in order to keep it up to the desired strength, and also to add water as it boils away. In this Way the bath is replenished from time to time, and continues in use, without entire renewal, for long periods of time.

When a bath is first prepared from fresh chemicals, the reaction is comparatlvely quick. After the bath has been used and re plenished for a considerable lengthof time in the manner described above, the time of reaction is very materially lengthened. The.

1929. Serial No. 359,166.

bath is said to be aged. It, requires additional timeor tank room, or both, to treat articles in an aged bath over what would be required in fresh solution, but in order tov avoid Waste-of chemicals, it has been considered necessary to continue to use aged baths. 1

Not only is the use of an aged bath troublesome-because of the increased length of time required in such a bath, but the-coating is in some Ways less desirable, and an article requires more careful and perfectcleaning in order to receive a satisfactory coating in an aged bath than it'does in a fresh bath. llhe action of the aged bath is particularly slow immediately at the start of the reaction. When articles are immersed in a fresh bath, reaction at a rapid rate begins at once. When the articles are'immersed in an aged bath, the. reaction is comparatively slow at the start, gradually increasing to a maximum, and finally decreasing gradually until the reaction is completed.

In accordance with the present invention, there are provided two baths, a'preliminary bath and a main bath. The preliminary bath is very much smaller than the main bath, and is prepared fresh with suflicient frequency to retain the characteristics of a new bath as distinguished from an aged bath. The articles 9 are then dipped in the preliminary bath for a brief period, merely sufficient to start the reaction, and are then immersed in the main bath and coated in the usual way. The main bath is replenished from time to time in the $5 usual'manner, sothat it is similar to the aged baths heretofore used.

The result of the preliminary treatment in a fresh bath is to greatly reduce the time necessary for the de siredreaction in the main 9 ings. To mention one specific example, when the time for producing a complete coating in the mainbath was upwards of an hour-for artioles not treated in the preliminary bath, it was found that articles given a one-mlnute 10 treatment in the preliminary bath reacted fully inthe main bath in one-half'hour, or

less than half of the time required for the articles without such preliminary treatment. The coating produced by this half hour treatment was as satisfactory in all respects as the coating produced by the longer reaction in the same bath'without the preliminary treat-.

ment'.

A'phosphate coating suificient for some purposes may be produced without leaving i the articles in the bath until chemical reaction ceases. In such cases, thereliminary treat-l ment may be employed wit a shortened time in the main bath.

or example, where a coatsuflicient for its intended use can be formed in an aged bath in one half hour, an

I equally good coating can be formed by a brieftity of material in the bath in proportion to the amount of articles treated. therein. For 'usual'practice, it has been liminary tank may be used for one day with found that the pie out renewal, and that then the entire contents which the preliminary roviding a preliminary bath'and a main ath large in'proportion to the preliminary bath, first treating articles for a short time in the preliminary bath andthereafter treating said articles in the-main bath for as long as is necessary to produce the desired coating, completely renewing the preliminary bath' at frequent intervals, and replenishing the main bath from time to time, regardless of its agmg 2. A method of producing a phosphate coating on metal articles which comprises providing a preliminary bath and a main bath large in proportion to the preliminary bath, first treating articles for a short time in the-preliminary bath and thereafter treating said articles inthe main bath for as long as is necessary to produce the desired coating,

transferrmgthe preliminary'bath into the Y main bath before he preliminary=.-bath ma teriall slows up in its action, an providing a fres preliminary bath.

In testimony whereof we have hereunto signed our names to this s ecification.

- I MATT EW GREENQ Y .VAN M. masnr.

of'the preliminary tank may be/transferred to the main tank, in order to avoid wast ng chemicals, and the preliminary tank may be replenished with a fresh bath. Inasmuch as i the main tank must be many times the size of the preliminary tank in order to fully treat v the normal boiling away' of the the articles,

* Y water in the main tank leaves ample room to receive therein the entire contents of the preliminary tank as often as it is necessary to empty the preliminary tank for renewal of the bath therein.

It will be understood also that where pre liminary bath and main bath are mentioned it is not necessary that the main bath be all in one tank, asit might be divided into .a plurality of tanks, and the articles treated in the preliminary tank might be removed therea from into difierent compartments of the main bath or in succession to one and then another of the compartments of the main bath. 'Y

The method is not limited by the examples given, for it" will be 'understoodthat the a the preliminary tank and Y the manner of. maintaining a fresh solution in the preliminary tank may be varied in ac-; v a v oordance with the"requirements and exvariations may e igencies of the articular case. lTherefore,

the appended claims.

What is claimed is :1

1. A method of produc' a phosphate 7 coating on-zmetal articles w "ch comprises ade within the scopeo'f Y 

